The technical definition of glatt kosher is meat from animals with smooth or defect-free lungs, but today the term glatt kosher often is used informally to imply that a product was processed under a stricter standard of kashrut, also referred to as the Jewish dietary laws.
For meat to be kosher, it must come from a kosher animal and be slaughtered in a kosher way. For meat to be glatt kosher, in addition to those two conditions, the meat also must come from an animal with adhesion-free or smooth lungs.
The word glatt means smooth in Yiddish. In Jewish dietary laws or kashrut, the term glatt is used to refer to the lungs of animals.
After the animal is slaughtered according to kashrut, the animal is opened and examined to determine whether the lungs are smooth.
If defects on the lungs are found, the meat is considered treif (torn, mortally injured, non-kosher). If the lungs are found to be defect-free or smooth, the meat is considered to be glatt kosher.
While the term glatt technically means the lungs of the kosher and kosher-slaughtered animal were smooth, the term often is used colloquially to imply a higher standard of kashrut, similar to the way the term mehadrin (food prepared in the strictest kosher way) is used in Israel.
Glatt Kosher Term Used Liberally
Even though only meat can be technically glatt kosher, the term is often loosely used today to refer to non-meat items.
Many suppliers of glatt kosher items will refer to all their products as glatt kosher. So one may find fish with the same glatt kosher sticker as is used on meat being sold one aisle over.
In addition, many suppliers of glatt kosher meat will refer to their whole service as glatt kosher. So don't be surprised to see caterers, restaurants, and stores labeled as glatt kosher.